1. Field of Invention
The present invention relates to a printing apparatus and a method thereof, and more particularly, to a multi-pass printing apparatus and a method thereof. The print data is divided into multiple passes of print data, and each pass of the print data is printed according to different delay position signals, such that a complete print output is achieved.
2. Related Art
A digital image stored in a computer usually comprises a mix of three primary colors: red (R), green (G), and blue (B) in different proportions. Taking a 24-bit image as an example, each of R, G, and B comprise 8 bits respectively. The color level of each primary color is 0˜255, and a color of black is displayed when the R, G, and B values are all 0, while a color of white is displayed when the R, G, and B values are all 255.
However, when a digital image in the computer is to be output, a problem occurs because many print and display devices cannot generate various color levels. Therefore, when a digital image is to be printed, the color must be converted to a color space of the output device at first. Taking a printer as an example, it is usually converted to C (cyan), M (Magenta), and Y (Yellow).
Then, the color level of the image is processed, such that an original multi-color level image is changed to a low-color level image. This step is referred to as halftoning. Each pixel of the halftone image is expressed by different degrees, and different degrees correspond to different numbers of ink droplets. Finally, the halftone image is converted to a print data to be output according to the arrangement of the ink holes of the ink-jet head, and finally a print image is produced.
Nowadays, with the progress of print systems and ink-jet heads, print resolution has been enhanced to more than 4800 DPI (Dots/Inch) from 300 DPI ten years ago. Generally speaking, a horizontal print resolution is mainly limited by motor control and the ink-jet head. In the print, the ink-jet head is controlled by the motor to move back and forth in a horizontal direction, and the motor sends a position signal at each position. The position signal is used to inform the ink-jet head when it is time to jet ink. At that time, the ink-jet driving mechanism sends a driving signal to the ink-jet head according to the print data, and then the ink-jet head jets ink droplets at the position.
Therefore, if the motor cannot meet the print resolution requirement, the ink droplets printed on the paper will deviate significantly, causing a degradation of the output quality. On the other hand, there is a limitation to the driving frequency of the ink-jet head, that is, the interval between two drives of a same ink-jet hole must be larger than a certain time interval. Otherwise, the ink-jet hole cannot be driven.
Furthermore, the size of the ink droplet is also an important factor limiting the print resolution. If a single ink-droplet exceeds the size of a print position, ink droplets will overlap, causing ink overflow, negatively affecting print quality when printing with a high resolution is carried out.
To solve the above disadvantages, U.S. Pat. No. 4,748,453 provides a print method with a checkerboard pattern, wherein the print data is divided into at least two passes to be printed. Through this print manner, adjacent dots in the horizontal and vertical directions are not be printed in a same pass, such that the ink droplets are prevented from staining each other and thus ink overflow or color shift is avoided.
A print method of a checkerboard pattern is also employed in U.S. Pat. No. 4,999,646. The difference lies in that the print passes are not completely overlapped, but partially overlapped. That is, a paper must be fed after a print pass has been finished, and then proceed to a next print pass, such that ink overlap or color shift can be avoided, and also the quality problem caused by incorrectness of the paper feed motor and breakdown of the ink-jet hole can be further reduced.
Furthermore, U.S. Pat. No. 5,633,663 provides another print method and device. A thinning mask is used to divide the original print data into two passes. Taking a relationship between different inks into consideration, the designed mask does not allow inks to be printed at a same pixel under a same pass. Therefore, ink overflow among different inks is improved.
The conventional techniques are all directed to avoid mutual staining of ink and color shift. Under high resolution printing, a specially-designed mask or pattern is used to divide the print data into different passes to be printed. A mask of a checkerboard pattern can be used to avoid the limitation of the driving frequency of the ink-jet head, but the horizontal print resolution is still the same as the data resolution, and horizontal control of the motor is still limited by the data resolution.